December 2017

What causes lizards’ brainsto change size?

Authors:Susan Crow, Meghan Pawlowski, Manyowa Meki,Lara LaDage, Timothy Roth II, Cynthia Downs,Barry Sinervo and Vladimir PravosudovAssociate editors: Lindsey Hall and Gogi Kalka

Мore free environmental science resources аt: www.ScienceJournalForKids.org 1

JUNE 2018

How can we keep the world free of measles?

Authors:Joaquin Prada, Jess Metcalf,Saki Takahashi, Justin Lessler,Andy Tatem and Matt FerrariAssociate editors: Lindsey Hall and Gogi Kalka

Have you had chickenpox? Do you remember how ill it made you feel? Add a fever and aching bones, and you’re close to how you’d feel if you had measles (Figure 1)...Chickenpox and measles are caused by viruses, and both are very contagious. This means they can spread very easily from one person to another. Measles can make you very sick, and people can even die from it.Luckily for us, measles is now very uncommon in countries with strong health care, because most people get vaccinated by doctors or nurses when they are young. However, not all

countries have good health care. In these countries measles still kills tens of thousands of children every year. Vaccination programs have reduced the number of cases, but it’s very hard to completely eliminate it and stop it from coming back.In this study, we created mathematical models to study how well different vaccination programs could work at preventing the disease from spreading through a country. We learned that under the right conditions, it can be possible to keep measles away for good.

Introduction

Abstract

All regions of the world have set goals to eliminate measles by 2020. They’ve chosen this disease because it kills thousands of children each year, and yet there is a cheap, safe and effective vaccine available which would allow us to eliminate it. Experts estimate that in the last 12 years, measles vaccination has prevented nearly 14 million deaths.To get to the point where there is no measles in the world, all countries must first get rid of the disease, and then prevent it from coming back.Measles vaccination protects against the disease. The vaccine consists of a small amount of the virus that has been weakened, which doctors then inject into our bodies. This allows our bodies to identify the virus and create special cells that can recognize, attack and kill the virus if we are exposed to it again, thus making us immune to the disease.

Measles vaccination is most effective when given as two doses. In countries with good healthcare, doctors give the first dose of routine vaccination to 1 year old babies, and the second dose to children between 3-5 years. After the first vaccination, less than 1 in 10 children (10%) are still at risk of getting the disease, but after the second, less than 1 in 100 (<1%) children are at risk. If nearly everyone gets vaccinated, there is something called

Figure 1: The measles virus causes an infection in the lungs, fever, and a red rash all over the body. It usually affects children.

We created mathematical models to represent four African countries with different sized populations and routine vaccination levels (Figure 2):

ethiopia – large population, medium vaccination level (81%)Nigeria – most populated African country, low vaccinationlevel (73%)Equatorial Guinea – small population, very low vaccinationlevel (43%)Swaziland – small population, high vaccination level (97%)

We used models to simulate the effects of adding a catch-up vaccination campaign every four years, starting in 2015 and continuing for 15 years. We looked at three different types of catch-up campaigns in each of the four countries:

vaccinating children up to 5 yearsup to 10 years or up to 15 years of age.

These catch-up campaigns are in addition to the routine (first dose) vaccination that each country does. Because vaccinating all children is very difficult, we looked at two realistic levels of coverage that could be achieved by the catch-up vaccination campaigns:

70% of target aged children were vaccinated90% of target aged children were vaccinated

We then looked at the effect of these different vaccination strategies on the likelihood of an outbreak of measles in the population when a country is close to having eliminated the disease.

JUNE 2018HoW CAn WE KEEP THE WoRLD FREE oF MEASLES?

Results

methods

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We found that we could keep measles out of a population for a short time after each round of catch-up vaccinations. This was the case even when the proportion of babies receiving routine vaccinations was low, as long as the catch-up campaign reached a large fraction of children and targeted a broad age range of children.However, we found that if the routine coverage was low, (few newborns were vaccinated) it was very hard to continuously

maintain high enough levels of population immunity to prevent a measles outbreak. There are just too many susceptible children before a catch-up campaign is carried out. This was the case even when a high proportion of children across a wide age range received the catch-up vaccination (Figure 3).In our models, giving the catch-up vaccinations to children between 10-15 years old didn’t significantly reduce the risk of an outbreak compared to only targeting children up to 10 years old.

population (or herd) immunity – enough people are immune, so it is unlikely that a sick person will come in contact with anyone who isn't immune, and thus the infection can't spread. Every child that does not get vaccinated makes it more likely measles could come back, so it is important to to have extra opportunities for children that have missed their routine doses to get vaccinated.Sub-Saharan Africa is where the majority of the world’s remaining measles burden is found, and routine vaccination

coverage is relatively low. This means that population immunity falls over time as there are more children that are susceptible to the disease. So countries make an extra effort, every few years, to do catch-up vaccinations for the children that did not receive the routine vaccine initially.In our study, we wanted to find out if there was a sustainable vaccination program that would create the right conditions for eliminating measles from a country.

Ethiopia

Nigeria

Swaziland

Equatorial Guinea

Figure 2: our models were based on these four countries in Africa

our study showed that without major improvements to routine health care, a program of catch-up vaccinations is needed to keep a country measles-free. While these catch-up vaccination programs can be very effective, they are also very expensive to run. They should therefore be used alongside improved routine vaccination programs.As healthcare improves in a country, the routine vaccination programs can reach more and more babies right away, although in some countries it hasn’t improved much recently. This can also be very hard to measure!We found that the best strategy to get the most out of catch-up campaigns in countries without a good routine vaccination

program is to try to vaccinate as many children of a wide age range as possible in the first catch-up campaign. Then it can be followed up by smaller scale and cheaper vaccination programs.Cost is an important consideration in planning vaccination programs. Countries therefore need to identify the most cost-effective strategy that keeps the virus from coming back.our study showed that if countries have to choose between spending resources to increase the age range of children getting the catch-up vaccinations, or to increase the vaccination coverage in a narrower age range, it was more effective to increase the coverage.

Vaccinations have saved the lives of millions of people from many diseases. They have even successfully eradicated (eliminated worldwide) one disease called smallpox. Several other diseases are close to being eradicated: measles, polio, mumps, rubella and guinea worm.

But we mustn’t become complacent when we have eliminated a disease. If not enough people continue to get the vaccine to maintain population immunity, there’s a risk of an outbreak if the disease gets reintroduced. That’s why it’s so important to follow the advice of your doctors and get vaccinated!

Discussion

Conclusion

JUNE 2018HoW CAn WE KEEP THE WoRLD FREE oF MEASLES?

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In the long term, the best strategy to keep a country free from measles is to increase the coverage of the routine vaccination.

Then do one large catch-up campaign followed by regular catch-up vaccination programs.

Figure 3: How does the risk of a measles outbreak decrease in our model country (similar to Ethiopia - with large population and medium vaccination levels)? Higher vaccination coverage = lower outbreak risk. Low risk can be achieved even if the catch up vaccination campaigns are done for older children (10 or 15 y.o.) but then it would take more doses(which is more expensive).

Total number of doses required if vaccinating children up to...

age 5 age 10age 15

70% coverage90% coverage

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Which campaign was more effective at preventing an outbreak of measles: 90% coverage of children by age 10, or 70% coverage of children by age 15?Which one required more doses of the vaccine?

JUNE 2018HoW CAn WE KEEP THE WoRLD FREE oF MEASLES?

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REFERENCESJ.M. Prada, C.J.E. Metcalf, S. Takahashi, J. Lessler, A.J. Tatem, M. Ferrari (2017)Demographics, epidemiology and the impact of vaccination campaignsin a measles-free world – Can elimination be maintained? Vaccine 35:1488–1493https://www.sciencedirect.com/science/article/pii/S0264410X1730172X

World Health organization: Measleshttp://www.who.int/mediacentre/factsheets/fs286/en/

Gates Foundation: Vaccination Deliveryhttps://www.gatesfoundation.org/What-We-Do/Global-Development/Vaccine-Delivery

Glossary of Key Terms

Catch-up vaccination - children in Africa generally received the measles vaccine at 9 months of age. Unfortunately, not all children get this routine vaccination and so we need to have large catch-up vaccinations campaigns to try to get children vaccinated that are up to 5,10 or 15 years of age. For some children, this will be a second dose of the same vaccine, if they received one as a baby, but for others it could be the first time that they receive a certain vaccination.

Contagious – we say that a disease is contagious if it can easily pass from an infected person to another person who is not immune to it. Measles is very contagious, and can easily spread – by a sneeze or cough, or sharing the same silverware.

Elimination – when a disease stops circulating in a region or country. Measles has been declared eliminated from the Americas by the World Health organization.

Eradication – when a disease is eliminated worldwide. Smallpox and rinderpest are the only diseases that has been eradicated so far.

Immunity – the ability of the body to recognize and fight a disease so that you don’t get sick from it.

Model – a representation of a system. A mathematical model represents real world situations using a variety of mathematical tools (e.g. graphs, equations, or diagrams).

Outbreak – a sudden and fast increase of people getting a particular disease which then quickly spreads through the population.

Population Immunity – when enough of the population gets vaccinated against a disease, we say that the population immunity is sufficient to prevent an outbreak, even if one or a few people were infected.

Routine vaccination – the first dose of a vaccine that is usually given to young children or babies.

Susceptible – when someone is able to be infected by a disease. If a person hasn’t been vaccinated against measles, they are susceptible to the disease.

Sustainable – able to be maintained at certain rate or level. In our case, we want the vaccination program to keep countries free of measles without being too expensive.

Vaccine/Vaccination – an injection of a killed or weakened pathogen, in order to stimulate the immune system against that pathogen (a pathogen is something that makes you sick). This means that the immune system can recognize those pathogens if they ever come back, and destroy or disable them, preventing disease.

Virus - an infective agent that typically consists of a nucleic acid molecule in a protein coat, is too small to be seen by light microscopy, and is able to multiply only within the living cells of a host.

JUNE 2018HoW CAn WE KEEP THE WoRLD FREE oF MEASLES?

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Check your understanding

Some people don’t want their children to receive the vaccination for measles (called the MMR vaccine, as it also protects you from two other diseases called mumps and rubella). What could happen if enough children don’t get the vaccine?

Why did increasing age range not help as much as increasing the coverage of the second vaccination programs?

our paper considered the simple costs of vaccination (the cost of the vaccine itself). The more doses of vaccine required, the higher the costs of the program. Beyond the cost of the vaccine, what other costs are there to a vaccination program?

Some countries find it easier than others to run successful vaccination programs (to get the vaccine to all of the ‘target’ children). What factors might make it hard for a country to run a successful vaccination program?

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